The present invention relates generally to echo cancelers, and more particularly to comfort noise generation for use with echo cancelers.
A communication channel generally has a near-end which transmits signals to a far-end. To prevent delayed or reflected far-end signals from interfering with a near-end listener, echo cancelers attempt to cancel any extraneous signals present on a channel. Because echo cancelers cannot ordinarily cancel all the unwanted signals on a channel, they also employ a center-clipping echo suppressor to suppress any residual echo on the channel.
Echo suppressors ordinarily operate by clipping portions of its output signal to zero. Many echo cancelers, however, do not insert any noise to replace the zero clipping of the echo suppressor. The result is a channel that suddenly sounds "dead" whenever the suppressor is active. To a far-end listener, these sudden variations in the noise level on the channel causes a choppy effect which impedes normal conversation. The zero clipping of the echo suppressor also causes a non-linear effect for vocoders which also degrades their performance. The sudden transition in levels introduces high frequency components into the signal which the vocoders cannot handle.
Some echo cancelers do employ a noise replacement technique in an attempt compensate for the effects of echo suppressors. Noise of either a fixed spectral shape (white or pink), or estimated spectral shape is inserted when the center-clipping echo suppressor is active. These techniques, however, also cause perceptible variations in the noise characteristics on the channel. The predetermined background noises, however, cannot accurately match the actual background noise which accordingly disturbs the listener. The effect becomes even more pronounced and objectionable when network delays are present, such as in satellite communication networks.
Therefore, there is a need for noise generation for use in echo cancelers to provide constant and continuous background noise to avoid perceptible variations in the noise characteristics on the channel.